A fluorescent technique was applied to study non-steady state grease lubrication in a ball-on-disk concentrated contact. Fluorescent grease was developed, which enabled dynamic behaviour of the base oil of the grease to be visually observed with an optical microscope and a normal CCD camera. Rolling and sliding tests were conducted under constant and reciprocating speed conditions. The fluorescence images clearly showed the occurrence of inlet oil starvation by repeated contact, where base oil was pushed out from the track leaving behind the thickener carrying less oil, but carrying gaseous cavities. In reciprocating rolling, oil starvation was generally less severe at slower speed, as in the case of uni-directional rolling. However, there was an obvious delay in oil recovery upon reversal of motion as compared to that in oil lubrication, and the delay appeared to be significant at higher acceleration. In reciprocating sliding, oil starvation was retarded due to a pileup of grease stagnating at the leading and trailing wedges of the conjunction.
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